The spectrograms are converted to low-level features by the convolution layer. There have been discussions how to define the convolution layer: use pre-defined model such as VGG-16 or random noise. VGG-16 has been successfully used by image style transfer [1]}, [2]} but [3]} argued that it was not as robust when applied to acoustic style transfer, because it was trained with images while spectrograms show different visual elements such as abstract-looking waves and shades and even reported random noises produce similar results. This claim also holds for transferring specific images where the styles are mostly material textures that are visually similar to random noises [4]}.
